1. Field of the Invention
The present invention relates to a semi-transmissive-type liquid crystal display device and a method for manufacturing the same and more particularly to the semi-transmissive-type liquid crystal display device having a plurality of pixel regions, each of which has a transmissive region and a reflective region and the method for manufacturing the same.
The present application claims priority of Japanese Patent Application Nos. 2002-201776 filed on Jul. 10, 2002, which is hereby incorporated by reference.
2. Description of the Related Art
A liquid crystal display device, thanks to its features being compact, thin, and low-power consuming, is becoming commercially practical in wide applications such as OA (Office Automation) equipment, portable cellular phones, or a like. Two types of methods for driving liquid crystal display devices including an active matrix method and a passive matrix method are known and the former that enables high quality display in particular is widely used. Moreover, such the liquid crystal display device that can be driven by the active matrix method is further classified into two types of the liquid crystal display devices, one being a transmissive-type liquid crystal display device and another being a reflective-type liquid crystal display device and both the two-types of liquid crystal display devices are operated based on a principle that a liquid crystal panel making up a main component of the liquid crystal display device serves as an electronic shutter to pass or intercept light fed from an outside to display an image and, therefore, the both have no self-emitting function, unlike in the case of a CRT (Cathode Ray Tube) display device, an EL (Electroluminescence) display device or a like. As a result, a liquid crystal display of any type separately requires a light source in order to display an image. For example, a transmissive-type liquid crystal display device is constructed so as to have a light source made up of a backlight source on a rear (that is, on a face opposite to an image display face) of the liquid crystal panel and so that a liquid crystal panel does switching between transmission and interception of light fed from the backlight source to control the display.
In such the transmissive-type liquid crystal display device as described above, a bright image can be obtained by receiving light fed from a backlight source all the time, irrespective of ambient brightness in places where the transmissive-type liquid crystal display device is used, however, its power consumption of the backlight source is generally large and a half of power of the transmissive-type liquid crystal display device is consumed by the backlight source, thus causing an increase in its power consumption. Especially, in the case of a transmissive-type liquid crystal display device that is driven by a battery, time during which the liquid crystal display can be used is short and, if a large-sized battery is employed in order to lengthen the time during which the liquid crystal display can be used, weight of an entire liquid crystal display device become large, causing an obstacle to making the device compact and lightweight.
To solve the problem of power consumption by a backlight source in a transmissive-type liquid crystal display device, a reflective-type liquid crystal display device is proposed which is constructed so that use of a light source is made unnecessary and light (ambient light) existing in a place surrounding the liquid crystal display device is used as a light source. The reflective-type liquid crystal display device is constructed so that a reflective plate is placed within a liquid crystal panel and displaying of an image is controlled in a manner that switching is done between transmission and interception of ambient light which has been fed into an internal portion of the liquid crystal panel and has been reflected off the reflective plate.
In the reflective-type liquid crystal display device, unlike in the case of the transmissive-type liquid crystal display device, since light from the backlight source is not required, it is possible to reduce its power consumption, to make it small-sized and lightweight. However, such the reflective-type liquid crystal display device has a problem in that, if it is dark in surroundings, ambient light does not serve sufficiently as a light source and therefore visibility is remarkably lowered.
Thus, each of a transmissive-type liquid crystal display device and a reflective-type liquid crystal display device has both merits and demerits. In order to obtain stable display, though light fed from a backlight source is effective, if only a backlight source is used as a light source, an increase in power consumption is inevitable.
To solve this problem, a conventional semi-transmissive-type liquid crystal display device is proposed, which is constructed so as to have both a transmissive region and a reflective region in a pixel region of a liquid crystal panel in order to reduce power consumption of a backlight source and to improve visibility even in the case of dark ambient light and so that operations as a transmissive-type liquid crystal display device and as a reflective-type liquid crystal display device can be performed by one liquid crystal panel.
Since such the semi-transmissive-type liquid crystal display device as described above has both the transmissive region and reflective region in the pixel region of the liquid crystal panel, even when ambient light is dark, by turning ON a backlight source and by using the above transmissive region, the semi-transmissive-type liquid crystal display device can be operated as the transmissive-type liquid crystal display device and a characteristic of high visibility that the transmissive-type liquid crystal display device provides can be fully utilized. On the other hand, when ambient light is fully bright, by turning OFF the backlight source and by using the above reflective region, the conventional semi-transmissive-type liquid crystal display device can be operated as the reflective-type liquid crystal display device and a characteristic of low power consumption that the reflective-type liquid crystal display device provides can be fully utilized.
In the conventional semi-transmissive-type liquid crystal device, light fed from a backlight source passes through a liquid crystal layer in the transmissive region used to have the conventional semi-transmissive-type liquid crystal device be operated as the transmissive-type liquid crystal display device and, on the other hand, incident light being ambient light travels and returns through the liquid crystal layer in the reflective-type liquid crystal display device used to have the conventional semi-transmissive-type liquid crystal display device be operated as the reflective-type liquid crystal display and, as a result, a difference in optical paths occurs between the incident light fed from the backlight source and the light being ambient light in the liquid crystal layer. Therefore, in the conventional semi-transmissive-type display device, as described later, unless a dimension of a gap (reflective gap) of a reflective region serving as a layer thickness of a liquid crystal layer and of a gap (transmissive gap) of a transmissive region also serving as, a layer thickness of the liquid crystal layer are set to be optimum values according to a twisted angle of the liquid crystal layer, intensity of outgoing light output from a display surface cannot be made optimum, due to a difference in retardation in the reflective region and in the transmissive region. Optimization of intensity of outgoing light in a transmissive region and a reflective region of a pixel region in the conventional semi-transmissive-type liquid crystal display device is described below.
[1] Optimization of Intensity of Outgoing Light in Transmissive Region and Reflective Region
FIG. 34 is a diagram showing an outline of configurations of a conventional semi-transmissive-type liquid crystal display device needed to optimize intensity of outgoing light in a transmissive region and in a reflective region. The conventional semi-transmissive-type liquid crystal display device, as shown in FIG. 34, includes an active matrix substrate 112, a facing substrate 116, a liquid crystal layer 117 being sandwiched between both the active matrix substrate 112 and the facing substrate 116, a backlight 118 being placed on a rear of the active matrix substrate 112, phase difference plates (λ/4, 4 plates) 120a and 120b being placed on an outside of each of the active matrix substrate 112 and the facing substrate 116, and polarizers 119a and 119b. Here, on a surface of the active matrix substrate 112 being opposite to the facing substrate 116 are placed a transmissive film 105 serving as a transmissive region in a pixel region and a reflective film 106 serving as a reflective region in the pixel region. Thus, by constructing the conventional semi-transmissive-type liquid crystal display device by arranging each of the components, it is made possible to control a state of polarization of incident light and outgoing light, as described later.
[2] Arrangement of Polarizer and Phase Difference Plate Placed in Upper Position
First, a case is explained in which the above conventional semi-transmissive-type liquid crystal display device is operated as a reflective-type liquid crystal device. A phase difference plate 120b is placed between the liquid crystal layer 117 and the polarizer 119b so that the reflective region is displayed in a normally white mode, that is, white display is made by a state in which liquid crystal molecules of the liquid crystal layer 117 lay themselves down (that is, liquid crystal molecules lie in a horizontal direction) due to no application of voltages between a facing electrode (not shown) of the facing substrate 116 and a pixel electrode (not shown) of the active matrix substrate 112 and so that black display is made by a state in which liquid crystal molecules stand up (that is, the liquid crystal molecules rise in a vertical direction) due to application of voltages between the facing electrode (not shown) of the facing substrate 116 and the pixel electrode (not shown) of the active matrix substrate 112. By placing the phase difference plate 120b in an manner so as to be rotated by 45° relative to an optical axis of the polarizer 119b, linearly polarized light (horizontal light) being ambient light having passed through the polarizer 119b becomes clockwise circularly polarized light. The clockwise circularly polarized light reaches the reflective light 106 as a linearly polarized light by setting a reflection gap “dr” at a specified value. The linearly polarized light is reflected off the reflective film 106, as it is, as linearly polarized light and becomes clockwise circularly polarized light when going out from the liquid crystal layer 117. The clockwise circularly polarized light is changed to be a linearly polarized light (horizontal light) by the phase difference plate 120b and goes out through the polarizer 119b having an optical axis in a horizontal direction and is displayed in a white mode. On the other hand, when a voltage is applied between the above facing electrode (not shown) and the above pixel electrode (not shown), liquid crystal molecules rise. At this point, light having incident on the liquid crystal layer 117 as clockwise circularly polarized light reaches the reflective film 106 as it is and is changed to be counterclockwise circularly polarized light by the reflective film 106 and is then reflected. The counterclockwise circularly polarized light, after having been emitted from the liquid crystal layer 117, is changed to be linearly polarized light (vertical light) by the phase difference plate 120b and is absorbed by the polarizer 120b without being emitted. This causes black display to be made.
[3] Arrangement of Polarizer and Phase Difference Plate Placed in Lower Position
Next, a case is explained in which the above conventional semi-transmissive-type liquid crystal display device is operated as a transmissive-type liquid crystal display device. Arrangement angle of an optical axis of each of the phase difference plate 120a and the polarizer 119a both being placed in a lower position is determined so that black display is made with a voltage being applied. The polarizer 119a in the lower position and the polarizer 119b in the upper position are placed in a manner so as to produce a cross Nicol relationship, that is, in a manner that the polarizer 119a in the lower position is arranged in a direction being rotated by 900 relative to the polarizer 119b. Moreover, in order to cancel (or compensate for) an influence by the phase difference plate 120b placed in the upper position, the phase difference plate 120a is placed in a manner that the phase difference plate 120a placed in the lower position is rotated also by 90° relative to the phase difference plate 120b placed in the upper position. Since liquid crystal molecules have risen while a voltage is being applied, a state of polarization of light remains unchanged. That is, a state in which liquid crystal molecules have risen with a voltage being applied is optically equivalent to a state in which the polarizer 119a and the polarizer 119b are placed in a manner. so as to produce a cross Nicol relationship between them, thus causing black display to be made with a voltage being applied. By configuring as above, arrangement of optical components and arrangement angle of an optical axis are determined in a liquid crystal panel in the conventional semi-transmissive-type liquid crystal display device.
[4] Setting of Twisted Angle
FIG. 35 shows a relation among a twisted angle (0° to 90°) of a liquid crystal, a reflection gap “dr” (layer thickness of a liquid crystal layer) and a transmissive gap “df” (layer thickness of a liquid crystal layer) in the conventional semi-transmissive-type liquid crystal display device configured by placing optical components at arrangement angles described above and by using a nematic liquid crystal having refractivity anisotropy “n” of 0.086 as the above liquid crystal layer 117. Moreover, FIG. 36 shows a relation among a twisted angles φ (0° to 90°), a transmittance, and a reflectivity obtained when the reflective gap “dr” and transmissive gap “dr” are optimized in the conventional semi-transmissive-type liquid crystal display device. In general, as a twisted angle becomes smaller, a usage rate of light in a transmissive mode becomes the higher and a color shift occurring when a field of view is swung becomes the larger. As is apparent from FIG. 35, when the twisted angle φ is about 72°, the reflective gap “dr” and the transmissive gap “df” are made equal to each other in which a reflectivity of white light and a transmittance of white light become maximum. Moreover, as the twisted anglesφ becomes smaller, the optimum reflective gap, “dr” becomes smaller than the optimum transmissive gap “df”.
As is apparent from FIG. 35, the optimum reflective gap “dr” and transmissive gap “df” are made equal to each other, both being about 2.7 μm, when a nematic liquid crystal having refractivity anisotropy □n of 0.086 is used and the twisted angle φ is set at about 72°. When the twisted angle φ is set at about 0°, a maximum reflective gap “dr” is about 1.5 μm and a maximum transmissive gap “df” is about 2.9 μm. When the twisted angle φ is set at about 60°, the maximum reflective gap “dr” becomes about 2.0 μm and the maximum transmissive gap “df” becomes about 2.8 μm.
As described above, to correct for a difference in optical paths of incident light passing through the transmissive region and the reflective region in the pixel region and to perform optimization on an intensity of outgoing light in the conventional semi-transmissive-type liquid crystal display device, it is necessary that an optimum reflective gap “dr” and optimum transmissive gap “df” that maximizes reflectivity and transmittance of white light, depending on a twisted angle of a liquid crystal, have to be set in a manner as shown in FIG. 35. Therefore, by placing a step as in the case of the conventional semi-transmissive-type liquid crystal display device as shown in FIG. 30 so that the reflective gap is made different from the transmissive gap and by forming the active matrix substrate 112 as in the case of the conventional semi-transmissive-type liquid crystal display device as shown in FIG. 33 so that the reflective gap is made equal to the transmissive gap, a contrivance to obtain an optimum reflective gap and an optimum transmissive gap, depending on a specified twist angle, has been conventionally used.
Configurations of a conventional semi-transmissive-type liquid crystal display device are described below by referring to FIG. 30. The semi-transmissive-type liquid crystal display device shown in FIG. 30 includes an active matrix substrate 112 on which a TFT (thin film transistor) is formed to operate as a switching element, a facing substrate 116, a liquid crystal layer 117 being sandwiched between the facing substrate 116 and the active matrix substrate 112, a backlight 118 placed on a rear of the active matrix substrate 112.
The active matrix substrate 112 includes a transparent insulating substrate 108, a gate line and a data line (not shown) being formed on the transparent insulating substrate 108, a gate electrode 101a connected to the gate line, a gate insulating film 109, a semiconductor layer 103a, a drain electrode 102a and a source electrode 102b drawn from both ends of the semiconductor layer 103a and connected respectively to the data line and a pixel electrode (not shown), and a passivation film 110. Also, the pixel region PX is divided into two portions, one being a transmissive region PXa to allow light fed from the backlight 118 to transmit and another being a reflective region PXb to reflect incident ambient light. In the above transmissive region PXa, a transparent electrode film 105 made of ITO (Indium Tin Oxide) or a like being formed on the passivation film 110. In the above reflective region PXb, a reflective electrode film 106a made of Al (Aluminum) or an Al alloy formed with a concave/convex shaped film 111 made of organic films or a like being interposed between the reflective electrode film 106a is formed in a manner to be connected to the transparent electrode film 105. The transparent electrode film 105 and the reflective electrode film 106a being connected to the source electrode 102b through a contact hole 107 formed on the concave/convex shaped film 111 operate as a pixel electrode (not shown). On the transparent electrode film 105 and the reflective electrode film 106a is formed an orientated film 129. Here, the TFT 103 is made up of the gate electrode 101a, gate insulating film 109, semiconductor layer 103a, drain electrode 102 and, source electrode 102b. On the other hand the facing substrate 116 includes a transparent insulating substrate 113, a color filter 114, a black matrix (not shown), a facing electrode 115, and the orientated film 105.
The semi-transmissive-type liquid crystal display device having such the configurations as shown in FIG. 30 operates in a manner that, in the transmissive region PXa, light from the backlight 118 which has entered from a rear of the active matrix substrate 112, after having passed the liquid crystal layer 117, is output from the facing substrate 116 and, in the reflective region PXb, ambient light which has entered from the facing substrate 116, after having passed through the liquid crystal layer 117, is reflected off the reflective electrode film 106a and again passes through the liquid crystal layer 117 and is then output from the facing substrate 116. By forming a step on the concave/convex shaped film 111 so that the reflective gap “dr” becomes a half of the transmissive gap “df” (however, in this case, the twisted angle φ is set at about 0°) and by making approximately equal lengths of optical paths of light passing through each of the transmissive region PXa and reflective region PXb, a polarization state of outgoing light is calibrated.
A reflective-type liquid crystal display device is disclosed in Japanese Patent Application No. 2001-221995 in which a transparent electrode is formed on a reflective plate having concave/convex portions with a protective film made of a transparent acrylic resin being interposed between the transparent electrode and the reflective plate. In the disclosed semi-transmissive-type liquid crystal display device, in order to solve a problem that, if a liquid crystal whose retardation is different between a transmissive display region and a reflective display region is oriented at a same driving voltage, a high-contrast display cannot be obtained and bright display is difficult, orientation of a liquid crystal is controlled after calibration has been made so that retardation in a portion to perform transmissive display and in a portion to perform reflective display is put into a near range. However, in the disclosed semi-transmissive-type liquid crystal display device, a countermeasure against a display defect caused by an electric erosion reaction and a flicker caused by a residual DC (Direct Current) voltage, which present a problem to be solved in the present invention, is not taken.
Moreover, in the disclosed semi-transmissive-type liquid crystal display device, since the reflective electrode film (reflective plate) is formed in a central portion of a pixel and a TFT device is not covered by the reflective plate, any countermeasure against problems handled in the present invention cannot be taken.
However, such the conventional semi-transmissive-type liquid crystal display device as described above has two problems. One problem (first problem) is that, in the conventional semi-transmissive-type liquid crystal display device, since the reflective electrode film 106a made of Al or an Al alloy is formed on the transparent electrode film 105 made of ITO, Al and/or ITO are eroded due to an electric erosion reaction when a resist pattern used to perform patterning on the reflective electrode film 106a is formed. Another problem (second problem) is that a flicker occurs due to a residual DC voltage produced in a region of the reflective electrode film 106a. 
The first problem of the electric erosion reaction is described first. For example, in such the conventional semi-transmissive-type liquid crystal display device as shown in FIG. 30, in order to connect the transparent electrode film 105 to the source electrode 102b in the TFT 103 through the reflective electrode film 106a, the transparent electrode film 105 and the reflective electrode film 106a are formed so as to overlap each other within each pixel, however, since electric separation is necessary between pixels adjacent to one another, overlapping between the transparent electrode film 105 in one pixel and the reflective electrode film 106a in other pixel being adjacent to the one pixel is not allowed. Therefore, as shown in FIG. 31A, when a resist pattern 121 used to form the reflective electrode film 106a is formed, only a reflective region side of a conductive film for the reflective electrode film 106a in each pixel already formed on entire surfaces of the pixel region PX has to be covered. However, as shown in FIG. 31B, if a crack occurs in the reflective electrode film 106a in an end portion (portion surrounded by broken lines in FIG. 31B) of the transparent electrode film 105 already formed due to some reasons, a developer 126 permeates the reflective electrode film 106a through this crack 127.
Since the Al material making up the reflective electrode film 106a is highly reactive and easily reacts with oxygen, if the developer 126 permeates through the crack 127 as described above, the Al material reacts with ITO being an oxide conductor which makes up the transparent electrode film 105. As a result, a reaction of erosion (oxidation) of Al and dissolution (reduction) of ITO of the developer 126 serving as an electrolytic solution, which are called an “electric erosion reaction”) occur which causes a contact failure between the Al and ITO and/or a peeled portion 128 between the poor-adhesive transparent electrode 105 and the passivation film 110 occurs. The electric erosion reaction is thought to occur due to a mechanism described below.    [A] An Al component having many lattice defect and impurity is dissolved as a local anode, causing formation of a pinhole.    [B] The developer 126 comes into contact with ITO contained in a lower layer through the formed pinhole.    [C] Oxidation of Al and reduction of ITO given by following expressions are stimulated by a potential between oxidation potential of Al and reduction potential of ITO in the developer 126 which serves as a driving force of reaction.Al+4OH−→4 H2AlO3+H2O+3e   (1)In2O3+3H2O+6e→2In+6OH−  (2)
Although the electric erosion reaction can be suppressed to some extent by taking a layout of the transparent electrode film 105 and the reflective electrode film 106a, that is, a state of overlapping of ITO and Al, into considerations, the electric erosion reaction is an essential problem in a structure in which an Al film or an Al alloy film is formed on ITO and, therefore, a proposal of a structure in which occurrence of the electric erosion reaction can be surely prevented is expected.
Next, the second problem of a flicker is explained. A semi-transmissive-type liquid crystal display device being driven by an active matrix method is ordinarily operated with an AC (alternating current) voltage and uses a voltage applied to its facing electrode as a reference voltage and feeds a voltage being changed to become a positive polarity and negative polarity in every period of time to its pixel electrode (pixel electrode) Though it is preferable that waveforms of a positive voltage, applied to a liquid crystal and of a negative voltage applied to the liquid crystal are symmetric to each other, even if AC voltages whose waveforms are symmetric are applied to its pixel electrode, waveforms of the voltage actually applied to the liquid crystal are not symmetric due to an unintentional DC component as described later. As a result, optic transmittance obtained when a positive voltage is applied is different from that obtained when a negative voltage is applied and luminance changes in a period of an AC voltage to be applied to a pixel electrode, causing a flicker to occur. As described later, this flicker occurs due to an orientated film 129 being formed on a face of each of the facing substrate 116 and active matrix substrate 112 placed on both sides of the liquid crystal layer 117 used to control orientation of a liquid crystal molecule.
As a material for the above orientated film 129, a polyimide resin is used because its mechanical strength is sufficient since rubbing processing is performed on the thin film with a thickness of about several-hundred angstroms, because the material has a resistance to solvents to be used in rinsing of the orientated films 129 with water or organic solvents after rubbing operations have been completed, and because the material has a resistance to heat which is generated when an epoxy resin used as a seal material is heated and cured at the time of sealing of the liquid crystal. However, it is known that the polyimide resin, when rubbing processing is performed thereon or when intense light is applied thereto, generates an electron within the polyimide resin.
In the semi-transmissive-type liquid crystal display device shown in FIG. 30, on the active matrix substrate 112 are formed the transparent electrode film 105 and the reflective electrode film 106a on which (on surfaces of sides of the liquid crystal layer 117 to be inserted) the orientated film 129 made up of polyimide is applied and, as described above, electrons are generated within polyimide due to the rubbing processing and/or application of light. Oxidation easily occurs on a surface of Al making up the reflective electrode film 106a and a Schottky barrier occurs at an interface surface between polyimide and the Al, making it difficult for electrons within polyimide to go out. On the other hand, since ITO making up the transparent electrode film 105 is not oxidized, the Schottky barrier does not occur at an interface surface between polyimide and ITO, thus allowing electrons deposited within polyimide to go out. As a results electrons reside only in polyimide making up the orientated film 129 on the reflective electrode film 106a and a residual DC voltage is produced. Since waveforms of a DC voltage to be applied to a pixel electrode (not shown) are not symmetric to one another due to existence of the DC component, a flicker occurs.
The second problem is also an essential problem in a structure in which, on an uppermost layer of the active matrix substrate 112 is formed the reflective electrode film 106a made of Al or a like on which the orientated film 129 made of polyimide is applied and a proposal of a structure in which occurrence of flickers caused by a residual DC voltage can be prevented is expected.